Cellular Reproduction: DNA Replication

Cellular Reproduction Basics

• All cells originate from pre-existing cells.
• New cells facilitate growth and replace old or damaged cells, leading to new organisms.
• There are two key processes: growth and division.
• Growth involves a cell duplicating its DNA and organelles.
• Division involves distributing the parent cell's DNA and contents to daughter cells.

DNA Replication

• DNA replication is when a cell copies its DNA.
• Chromosomes replicate just before cell division.
• Each chromosome creates a copy of itself, which includes its DNA.
• A full DNA copy is passed to both daughter cells after cell division.

Chromosome Components

• Chromatin is an uncoiled thread composed of DNA and proteins.
• During cell division, chromatin condenses to form eukaryotic chromosomes.
• Chromatin is spaghetti-like in appearance.
• Chromatin is wound around histones
• Histones are protein molecules aiding DNA organization.
• Nucleosome: Each "bead" within chromatin
• Before division, looped chromatin compacts chromosomes.

Chromosomes

• Chromosomes undergo DNA replication and are duplicated before cell division.
• Duplicated chromosomes consist of two identical sister chromatids.
• Sister chromatids are held together by the centromere.
• Each sister chromatid has an identical DNA double helix.
• Chromosomes are found in the nucleus.
• Somatic cells, or body cells, contain 46 chromosomes or 23 pairs.
• Gametes, or sex cells, contain 23 chromosomes.

Chromosome Number Alterations

• Humans can be born with abnormal chromosome numbers due to a failure in chromosome separation.
• Nondisjunction: The failure of chromosomes to separate correctly, resulting in an abnormal number of chromosomes.

The Cell Cycle

• The cell cycle has staged to facilitate cell division and produce new cells through interphase, mitosis, and cytokinesis.

Interphase

• A phase where the cell performs its usual functions.

Mitosis

• Mitosis (mitotic division) includes nuclear division.
• Cell division happens only here.
• Results in two identical daughter cells, which can be asexual or sexual.

Cytokinesis

• Division of the cytoplasm.

Cell Cycle Regulation

• Cyclins act as internal signals, serving as cellular timekeepers during particular cell cycle stages.
• Kinases facilitate cellular activities (removal of the nuclear membrane) by removing a phosphate from ATP and adding it to other molecules.
• The cell cycle lasts 24 hours.
• Interphase takes 22 hours.
• The mitotic phase takes 2 hours.
• Contact inhibition: Cell division continues until cells are in close proximity to each other.

Interphase Stages

• Consists of a nuclear envelope, nucleolus, centrioles, and chromatin
• It has three phases: G1, S, and G2.

G1 Phase

• The G1 phase is for growth.
• It acts as a checkpoint to decide whether the cell will die or divide.
• It marks the beginning of interphase, where the cell grows.
• Cells are either corrected or eliminated in this phase if they are damaged; if not, they can cause cancer later.

S Phase

• The S phase carries out DNA synthesis and replication.

G2 Phase

• The G2 phase verifies DNA replication and acts as a checkpoint.
• The cell will arrest for repair if DNA is damaged.
• Organelles are created for cell division.

Mitotic Checkpoint

• It takes place between metaphase and anaphase.
• It ensures chromosomes properly attach to the spindle and distribute accurately to the daughter cells.
• Apoptosis: Programmed cell death breaks cells into fragments.
• An example is when a tadpole becomes a frog and its tail disappears due to apoptosis.
• An example in humans is how fingers and toes are webbed and then freed from one another by apoptosis.

Mitotic Phase (PMAT)

• This phase has four stages: prophase, metaphase, anaphase, and telophase.

Prophase

• Miotic spindles appear during prophase.
• Under a microscope, chromosomes are visible in condensed (visible) pairs.
• In prophase, the mitotic spindle nuclear envelope separates sisters and mitotic spindle fibers.
• The nuclear envelope/membrane disintegrates, and the nucleus disappears.
• Chromosomes condense.
• Sister chromatids attach at the centromere.

Metaphase

• Sister chromatids align along the equatorial (middle) plate or metaphase plate.
• M = middle

Anaphase

• Sister chromatids are pulled to opposite poles.

Telophase and Cytokinesis

• Two distinct cells are visible under the microscope.
• Two new daughter nuclei are formed.
• Membrane returns to interphase during telophase.
• The nucleolus comes back; chromosomes decondense.
• In animal cells, cytokinesis involves separation and cleavage furrow formation.

Cytokinesis

• Animal cells undergo cleavage furrow formation.
• Plant cells undergo cell plate formation.
• Nucleoli are present.
• Centrioles are only found in animal cells within a centrosome with collections of microtubules.
• Centrosomes: Organizing center contains two centrioles for the mitotic spindle with collections of microtubules.

Meiosis

Karyotype

• A karyotype is an image of chromosomes arranged by size, shape, and banding pattern.
• Chromosomes exist in pairs with one of each chromosome inherited from each parent.
• There are 22 pairs of autosomes.
• A 23rd pair of sex chromosomes

Chromosome Numbers

• Males and females have 23 pairs of chromosomes.
• Diploid (2n): 46 chromosomes in total, or two sets of chromosomes.
• All body cells are diploid.
• Haploid: Half the chromosomes, or 23, which is one set of chromosomes.
• In human gametes (sperm and ova).
• Females have XX chromosomes which are larger.
• Males have XY chromosomes which are smaller.

Homologous Chromosomes

• They exist as chromosome pairs.
• They have the same size, shape, and centromere location.
• Each parent contributes one homologous chromosome to each pair.

Tetrads

• These are groups of four sister chromatids which are replicated copies of chromosomes.
• Tetrads facilitate crossing over in meiosis I.
• Occurs in meiosis I.

Crossing Over

• It takes place in prophase I.
• DNA is exchanged between non-sister chromatids.

Human Life Cycle

• All reproductive events from one generation to the next.
• It involves meiosis and mitosis.
• Spermatogenesis: Produces sperm in the testes.
• Oogenesis: Produces eggs in the ovaries.
• A haploid sperm fertilizes a haploid egg to form a diploid zygote.
• The zygote then undergoes mitosis, developing into a newborn.
• Mitosis continues for growth and repair throughout life.

Order of the Human Lifecycle

• Human life cycle + Meiosis + haploid gametes + fertilization + diploid cell + mitosis = a new individual, and then the cycle repeats.

Meiosis

• There are two divisions that result in four daughter cells.
• It consists of two parts: meiosis I and meiosis II.

Meiosis I

• Homologous pairs line up during synapsis and create a tetrad.
• Paired homologous chromosomes cross over and separate, ending in two haploid cells.

Meiosis II

• Haploid + haploid = diploid .
• Sister chromatids separate = four haploid cells.
• There is no chromosome duplication or interphase.
• Chromosomes have two sister chromatids.
• There are two daughter nuclei separate.
• This creates four new daughter cells, which have a haploid number of chromosomes (genetically different gametes).

Meiosis I: Interphase

• Chromosomes replicate.
• Two identical sister chromatids are held together at the centromere.

Meiosis I: Prophase I

• The nuclear envelope disintegrates separating the nucleoplasm from the cytoplasm.
• The nucleolus disappears
• Chromosomes condense.
• Synapsis occurs between homologous pairs and results in tetrad pairs with each homolog having two sister chromatids.
• Crossing over of nonsister chromatids
• Spindle formation

Meiosis I: Metaphase I

• Tetrads line up on the equator.
• They attach to kinetochore microtubules.

Meiosis I: Anaphase I

• Homologous pairs separate and move to opposite sides of the cell.

Meiosis I: Telophase I and Cytokinesis

• Haploid daughter cells form with two sister chromatids.
• Cytokinesis facilitates the formation of haploid daughter cells and cleavage furrow creation.
• Then reformation of nuclear envelope

Meiosis II Steps

• Same as haploid, except cells are now haploid instead of diploid

Meiosis II: Prophase II

• Spindle apparatus forms.
• Nuclear envelope fragments.
• Nucleolus disappears.
• Each still has chromatids attached to the spindle.

Meiosis II: Metaphase II

• The cell lines up on the spindle equator.
• The 2 sister chromatids are not genetically identical.
• Sister chromatid kinetochores attach to microtubules.

Meiosis II: Anaphase II

• Sister chromatids separate and move to opposite poles.

Meiosis II: Telophase I and Cytokinesis

• Nuclear envelope forms and the spindle forms.
• Four haploid gametes with different combinations of chromosomes and genetically dissimilar

Meiosis vs. Mitosis

• Meiosis involves two consecutive nuclear divisions, while mitosis involves one nuclear division.
• Meiosis= 2 phases (I & II) // Mitosis = 1 phase
• Synapsis happens in prophase I during meiosis
• Meiosis: 4 haploid cells // Mitosis = 2 diploid cells
• Meiosis = genetically identical // Mitosis = genetically different, which is crossing over

Syndromes

• Turner syndrome occurs in females when one of their X chromosomes is missing or partially deleted resulting in an XO chromosome.
• Klinefelter syndrome results in males having an extra X chromosome resulting in XXY.
• Down syndrome/Trisomy 21 : Includes one extra chromosome 21 (3 instead of normal 2)

Genetics

• Punnett squares visualize every possible combination gametes may inherit
• Original parents = P generation
• First-generation offspring = F1 generation
• Second-generation offspring = F2
• Monohybrid has two individuals with different alleles for a single trait like AA/aa/Aa

Mendel’s Law

• Each individual has two factors for each trait
• Factors separate in gamete formation
• Each gamete contains only one factor from each pair of factors.
• Fertilization gives each new individual two factors for each trait.

Testcross

• Cross between an individual and an unknown genotype with an individual with the homozygous recessive genotype

Genes

• Heritable factor that controls a certain characteristic, like eye color

Alleles

• These are alternative forms of the same gene like in blue and brown eyes.
• Location = locus

Dominant Traits

• Determines the appearance
• Uppercase/AA/Aa

Recessive Traits

• Has no noticeable effect.
• It shows if the individual has two copies that are lowercase in aa/Aa

Genotype

• This is the genetic makeup (words).
• Alleles that the individual receives at fertilization.
• Ex. AA
• Homozygous: Two identical alleles.
  • Homozygous recessive = aa
• Homozygous dominant = AA
• Heterozygous: Two different alleles = Aa

Phenotype

• An individual's trait, like having black fur.

Two-Trait Inheritance

• Genes segregate into two traits during gamete formation
• All possible combination of factors can happen in the gametes.
  • This is a Dihybrid cross.
• Dihybrid cross = FOIL: First, Outside, Inside, Last

Disease

• Sickle-cell: Autosomal recessive order where the gene changes to change amino acid in hemoglobin.
  • Causes a change in sickle shape and leads to poor blood flow and low resistance.
• Huntington: Autosomal Dominant where the nervous system degenerates.

Beyond Mendel's Law

• The traits are not always dominant or recessive/ either or.
• Most traits occur on a spectrum.

Incomplete Inheritance

• Heterozygous can have intermediate phenotypes such as wavy hair.

Multiple Allele Traits

• ABO-blood has 3 alleles.
• There is codominace and the results are not either or,

Polygenetic

• Controlled by 2+ sets where a quantitative effect on the phenotype is added cumulatively/ additive.
  • Multifactorial traits can be affected polygenetically

Polygenetic Inheritance- Environmental

• A trait or gene is affected by the environment.
• In response to radiation it can cause a change in skin.

Pleiotropy

• Single genes have more than just one effect, illness like disease of cells.
• Marfan syndrome: Production of an abnormal connective tissue
• Ex. Sickle cell, Anemia/ Porphria

Polygenetic

• There are many traits like skin or height.

Sex Linked Inheritance

• Individual of sex has chromosomes of X/Y of the Zygote and eggs and sperm.
• An egg can have multiple cells.
  • X is larger and has more genes.
• A linked chromosome is a gene that is carried of the X chromosomes.
• Sperm can also determine the sex/ or determine the child.
• Y carriers SRY- Maleness
• Most gene disorders are either missing or not there.
• Blood has mulitple traits!

DNA

• Carries genetic informatoin.
• A bonds with T and G binds with C ( AT-GC)
• It has double strands.
• DNA makes a copy of NDA while transcription makes MRNA DNA is more effective than RNA.
• Rosalinda's xray stated that DNA was electrical pattern that repeats.
• DNA has single and multiple strand ( A binds to U AND G Binds to C (AU-GC).
• The semiconversitive model is a double set between multiple strands.